WASHINGTON – President Donald Trump, who in the past has promoted the theory that vaccines cause autism, said Friday that unvaccinated children should be immunized in light of measles outbreaks in several states.

“They have to get those shots,” Trump told reporters on the White House South Lawn as he was leaving for Indianapolis, where he was to address a National Rifle Association convention. “The vaccinations are so important.”

Trump’s comments come as the number of reported cases of measles in the United State this year already has surpassed previous annual totals this century – less than four months into 2019, according to the U.S. Centers for Disease Control and Prevention.

Measles was declared eliminated from the United States in 2000. But 695 cases, mostly concentrated in New York City, already have been reported this year.

In California, hundreds of students and staff have been quarantined at the University of California, Los Angeles and California State University, Los Angeles because of possible measles exposure.

President Donald Trump(Photo: SAUL LOEB, AFP/Getty Images)

Trump has a long history of comments urging caution on vaccines. Before he ran for president, he incorrectly suggested they may lead to autism. In more recent years, Trump has said he supports children getting vaccinations but argued the shots should be more spread out over time.

“Healthy young child goes to doctor, gets pumped with massive shot of many vaccines, doesn't feel good and changes — AUTISM. Many such cases!” the president tweeted in 2014, drawing backlash from public health experts.

During his campaign for president in 2016, Trump drew additional attention for meeting with prominent anti-vaccine proponents, but he has insisted in the past that he supports vaccines generally, just not the schedule on which they are given.

“I'm not saying to not give vaccines, I am just saying give them small doses over a long period of time – not one massive dose for a child,” Trump tweeted in 2014.

To all haters and losers: I am NOT anti-vaccine, but I am against shooting massive doses into tiny children. Spread shots out over time.

Numerous studies have discredited the theory of a link between autism and vaccinations. Official groups, including the CDC, have reported there is no proven link between vaccines and autism and no ingredients in vaccines that could cause autism. Still, a minority of parents have chosen not to vaccinate their children.

Vaccine hesitancy, defined as the reluctance or refusal to be vaccinated or to vaccinate your children, has been identified by the World Health Organization as one of the top 10 global health threats of 2019.

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President Donald Trump and first lady Melania Trump watch the kids participate in the 141st annual White House Easter Egg Roll on the South Lawn of the White House, April 22, 2019. Jack Gruber, USA TODAY NETWORK

President Donald Trump and first lady Melania Trump blow whistles to start groups of children participating in the 2019 White House Easter Egg Roll, April 22, 2019, on the South Lawn of the White House. Jack Gruber, USA TODAY

President Donald Trump, joined by the Easter Bunny and first lady Melania Trump, speaks from the Truman Balcony of the White House, April 22, 2019, to start off the 141st White House Easter Egg Roll. Andrew Harnik, AP

First Lady Melania Trump blows the whistle standing with President Donald Trump during the annual White House Easter Egg Roll on the South Lawn of the White House, April 22, 2019. JIM WATSON, AFP/Getty Images

Two kids wait for the arrival of President Donald Trump and first lady Melania Trump to start the annual White House Easter Egg Roll on the South Lawn of the White House, April 22, 2019. JIM WATSON, AFP/Getty Images

The 2019 White House Easter Egg Roll on the South Lawn of the White House in Washington, April 22, 2019. This years 141st annual tradition is expected to draw more than 30,000 people. ERIK S. LESSER, EPA-EFE